The present invention relates to a sensor supporting structure for an accelerometer.
In general, the sensor of an accelerometer is housed and supported in a cylindrical housing and mounted on an object for which an acceleration input thereinto is detected or measured. FIG. 1 shows the construction of this kind of sensor supporting structure 10 disclosed in Japanese Patent Application Laid-Open No. 311173/88. In a sensor part 11 of a cylindrical outward appearance, there is housed, for example, a known capacitance pick-off type acceleration sensor which detects, as a capacitance change, a displacement of a quartz plate spring which is caused by an input acceleration, or a known piezoelectric voltage detection type acceleration sensor which detects a voltage resulting from the deflection of a quartz plate caused by acceleration. The acceleration that is measured which is applied in the axial direction A of the sensor. A housing 12 is composed of a cylindrical portion 13 and a flange 14 formed integrally therewith on the outer periphery thereof, and the sensor part 11 is received in the cylindrical portion 13 coaxially therewith. There is provided a predetermined gap between the inner peripheral surface of the cylindrical portion 13 and the outer peripheral surface of the sensor part 11, and the cylindrical portion 13 and the sensor are fixed by a ring 15.
The acceleration sensor supporting structure 10 has its cylindrical portion 13 received in a circular hole 17 made in a mount 16 with the flange 14 resting on the mount 16 along the marginal edge of the hole 17 and fixed thereto by screws 19.
Under environmental conditions in which the mount 16 undergoes a drastic temperature change, a thermal stress is produced between the mount 16 and the housing 12 of the sensor supporting structure 10 due to a difference between their thermal expansion coefficients. The conventional sensor supporting structure 10 has an arrangement which prevents the thermal stress from being transmitted to the sensor part 11 and adversely affecting its operation.
Since the ring 15 for fixing the sensor part 11 to the housing 12 is located apart from the flange 14 of the housing 12 in its axial direction A as shown in FIG. 1, the thermal stress generated between the mount 16 and the flange 14 is not directly transmitted to the sensor part 11 but instead it is absorbed by an elastic deformation of the cylindrical portion 13 of the housing 12 as indicated by the broken lines in FIG. 2.
Moreover, it is conventional, with a view to mounting the sensor supporting structure 10 on the mount 16 with a high degree of accuracy, that the gap G1 between the interior surface of the circular hole 17 of the mount 16 and the outer peripheral surface of the cylindrical portion 13 of the support structure 10 is set to a minimum as long as the cylindrical portion 13 can be inserted in the circular hole 17.
Thus, the sensor supporting structure 10 of this kind is so designed as to prevent measurement of acceleration from being influenced by ambient temperature.
On the other hand, in the case of boring the earth vertically in oil-well drilling or the like through utilization of the fact that detected gravitational acceleration decreases as the axial direction of the sensor part 11 (the direction of the detection of acceleration) deviates from the vertical direction, an accelerometer is mounted on the drill to measure the gravitation acceleration. In an environment in which the mount 16 is subject to a great impact of, for example, 2000G, the conventional sensor supporting structure 10 is defective in that, when it is exposed to a great impact in its radial direction, the quartz plate held in the sensor part 11 is readily broken. The sensor supporting structure 10 is mounted by the flange 14 on the mount 16 as if it is suspended therefrom. Now, let it be assumed that an impact is applied to the sensor supporting structure 10 in a direction B perpendicular to the axial direction A of the sensor part 11. When the impact acceleration is small, it is attenuated by the cylindrical portion 13 of the housing 12 as in the case of the aforementioned absorption of thermal stress. When the impact acceleration is large, the situation occasionally arises where the cylindrical portion 13 collides against the mount 16 as depicted in FIG. 3 because the gap G1 between the circular hole 17 and the cylindrical portion 13 is small and a secondary impact is generated by the collision and transmitted to the sensor part 11, resulting in breakage of the quartz plate (not shown) in the sensor part 11.
The secondary impact by the collision is complex in direction as compared with the impact acceleration applied to the mount 16, and in the case where the impact acceleration is continually applied and the secondary impact is also continually caused and accelerations are superimposed, the resulting secondary impact acceleration becomes greater than the applied impact acceleration.
One possible solution to this problem is to increase the gap G1 between the circular hole 17 and the cylindrical portion 13 in FIG. 1 to avoid the collision of the cylindrical portion 13 against the mount 16 due to the impact acceleration. In this instance, however, since the accuracy of mounting the support structure 10 on the mount 16 is impaired and since it is difficult to machine the mounting surface 18 of the mount 16 completely flat, the axial direction A of the sensor part 11 changes relative to the direction of acceleration to be measured each time the sensor supporting structure 10 is mounted and dismounted, that is, the reproducibility of the mounting position of the structure 10 is impaired, making it impossible to measure acceleration with accuracy.